A client device can render a web page for display on a monitor or screen of the client device. The client device often only displays a portion of the rendered web page in the finite display area of the monitor. The web page may contain one or more nested elements or frames such as a video player. The video player may, from time to time, display overlay content over a portion of the video. In some cases it may be desirable to measure whether or not the client device has displayed an overlay content item on the monitor.